1. Field of the Invention
The present invention relates to an image forming apparatus employing an LED (light emitting diode). More particularly, the present invention relates to an image forming apparatus such as a digital electrophotographic copying machine, a printer, etc.
2. Description of the Prior Art
For digital image forming apparatuses, an LED head is frequently used as exposure means for writing an image onto the photoreceptor surface which has already been charged. The LED head has a structure where pairs consisting of an LED device and SELFOC lens are linearly arranged in a direction parallel to the axis of a photoreceptor drum (that is, in a direction along the width of the photoreceptor drum). The number of pairs corresponds to the number of picture elements in the direction. Although the LED head is arranged in a direction along the axis of the photoreceptor drum as described above, the arrangement position of the LED head sometimes shifts with respect to the direction along the width of the photoreceptor because of a thermal expansion of the whole LED head caused by a light emission of the LED device. This means that a formed image is shifted from an intended position corresponding to an original, which shift is undesirable. However, in a conventional image forming apparatus for paper of smaller size, since the number of LED devices and the thermal expansion are small so that hardly any problems are caused, both ends of the LED head are fixed along the width of the photoreceptor on a supporting base substance to which the photoreceptor is attached.
In an image forming apparatus for paper of larger size, since a greater number of LED devices are required because of a large number of the picture elements, an expansion and contraction, in actual size, of the LED heads and the shift of a formed image from an intended position corresponding to an original due to the thermal expansion cannot be ignored, so that, for example, there are occasions when the center of the original and that of the formed image do not correspond.